Diabetes, a disorder of carbohydrate metabolism, has been known since antiquity. Diabetes results from insufficient production of or reduced sensitivity to insulin. The insulin molecule consists of two chains of amino acids linked by disulfide bonds (mw 6,000). The β-cells of the pancreatic islets secrete a single chain precursor of insulin, known as proinsulin. Proteolysis of proinsulin results in removal of four basic amino acids (numbers 31, 32, 64 and 65 in the proinsulin chain: Arg, Arg, Lys, Arg respectively) and the connecting (“C”) polypeptide. In the resulting two-chain insulin molecule, the A chain has glycine at the amino terminus, and the B chain has phenylalanine at the amino terminus.
Insulin may exist as a monomer, dimer or a hexamer formed from three of the dimers. The hexamer is coordinated with two Zn2+ atoms. Biological activity resides in the monomer. Although until recently bovine and porcine insulin were used almost exclusively to treat diabetes in humans, numerous variations in insulin between species are known. Porcine insulin is most similar to human insulin, from which it differs only in having an alanine rather than threonine residue at the B-chain C-terminus. Despite these differences most mammalian insulin has comparable specific activity. Until recently animal extracts provided all insulin used for treatment of the disease. The advent of recombinant technology commercially available from Eli Lilly and Company, Indianapolis, Ind.).
Insulin is necessary for normal utilization of glucose by most cells in the body. In persons with diabetes, the normal ability to use glucose is inhibited, thereby increasing blood sugar levels (hyperglycemia). As glucose accumulates in the blood, excess levels of sugar are excreted in the urine (glycosuria). Other symptoms of diabetes include increased urinary volume and frequency, thirst, itching, hunger, weight loss, and weakness.
There are two varieties of diabetes. Type I is insulin-dependent diabetes mellitus, or IDDM. IDDM was formerly referred to as juvenile onset diabetes. In IDDM, insulin is not secreted by the pancreas and must be provided from an external source. Type II adult-onset diabetes can ordinarily be controlled by diet although in some advanced cases insulin is required.
Before the isolation of insulin in the 1920s, most patients died within a short time after onset. Untreated diabetes leads to ketosis, the accumulation of ketones, products of fat breakdown, in the blood; this is followed by acidosis (accumulation of acid in the blood) with nausea and vomiting. As the toxic products of disordered carbohydrate and fat metabolism continue to build up, the patient goes into diabetic coma.
Treatment of diabetes typically requires regular injections of insulin. The use of insulin as a treatment for diabetes dates to 1922, when Banting et al. (“Pancreatic Extracts in the Treatment of Diabetes Mellitus,” Can. Med. Assoc. J., 12: 141–146 (1922)) showed that the active extract from the pancreas had therapeutic effects in diabetic dogs. Treatment of a diabetic patient in that same year with pancreatic extracts resulted in a dramatic, life-saving clinical improvement. Due to the inconvenience of insulin injections, massive efforts to improve insulin administration and bioassimilation have been undertaken.
Attempts have been made to deliver insulin by oral administration. The problems associated with oral administration of insulin to achieve euglycemia in diabetic patients are well documented in pharmaceutical and medical literature. Digestive enzymes in the GI tract rapidly degrade insulin, resulting in biologically inactive breakdown products. In the stomach, for example, orally administered insulin undergoes enzymatic proteolysis and acidic degradation. Survival in the intestine is hindered by excessive proteolysis. In the lumen, insulin is barraged by a variety of enzymes including gastric and pancreatic enzymes, exo- and endopeptidases, and brush border peptidases. Even if insulin survives this enzymatic attack, the biological barriers that must be traversed before insulin can reach its receptors in vivo may limit oral administration of insulin. For example, insulin may possess low membrane permeability, limiting its ability to pass from the lumen into the bloodstream.
Pharmaceutically active polypeptides such as insulin have been conjugated with polydispersed mixtures of polyethylene glycol or polydispersed mixtures of polyethylene glycol containing polymers to provide polydispersed mixtures of drug-oligomer conjugates. For example, U.S. Pat. No. 4,179,337 to Davis et al. proposes conjugating polypeptides such as insulin with various polyethylene glycols such as MPEG-1900 and MPEG-5000 supplied by Union Carbide.
U.S. Pat. No. 5,567,422 to Greenwald proposes the conjugation of biologically active nucleophiles with polyethylene glycols such as m-PEG-OH (Union Carbide), which has a number average molecular weight of 5,000 Daltons.
U.S. Pat. No. 5,359,030 to Ekwuribe proposes conjugating polypeptides such as insulin with polyethylene glycol modified glycolipid polymers and polyethylene glycol modified fatty acid polymers. The number average molecular weight of polymer resulting from each combination is preferred to be in the range of from about 500 to about 10,000 Daltons.
Polyethylene glycol is typically produced by base-catalyzed ring-opening polymerization of ethylene oxide. The reaction is initiated by adding ethylene oxide to ethylene glycol, with potassium hydroxide as catalyst. This process results in a polydispersed mixture of polyethylene glycol polymers having a number average molecular weight within a given range of molecular weights. For example, PEG products offered by Sigma-Aldrich of Milwaukee, Wis. in are provided in polydispersed mixtures such as PEG 400 (Mn 380–420); PEG 1,000 (Mn 950–1,050); PEG 1,500 (Mn 1,400–1,600); and PEG 2,000 (Mn 1,900–2,200).
It is desirable to provide non-polydispersed mixtures of insulin-oligomer conjugates where the oligomer comprises polyethylene glycol.